Flash sintering of ultra‐high pure alumina ceramics with fine microstructure

Abstract

The nearly fully dense ultra‐high pure (>99.99%) α‐Al2O3 ceramics were prepared by flash sintering at the furnace temperature of 1300°C. Compared with the samples sintered at 1300 and 1650°C without electrical field, the flash‐sintered samples exhibited remarkably improved density and finer grains. The flash‐sintered samples also exhibited high hardness, which is even higher than that of the hot‐pressed sample. Therefore, it is believed that flash sintering could be an effective technique for the preparation of ultra‐high pure alumina ceramics.

Densification behaviors of Al2O3 ceramics during flash sintering

“…This could be understood as follows: () As for conventional sintering, average grain size in the samples sintered to similar densification under different heating rates is assumed to be similar, 34 but in FS, the grain size slightly increased with current density even at same holding time. () The applied electrical field can inject the extra oxygen vacancy ($$V_{\rm{O}}^{^{\prime\prime}}$$) into alumina ceramics 26 . The abundant $$V_{\rm{O}}^{^{\prime\prime}}$$ combined with the Joule effect may result in a lower densification activation energy than that in the pure thermally activated densification process 33 …”

“…(2) The applied electrical field can inject the extra oxygen vacancy (𝑉 ′′ O ) into alumina ceramics. 26 The abundant 𝑉 ′′ O combined with the Joule effect may result in a lower densification activation energy than that in the pure thermally activated densification process. 33…”

“…The effects of initial voltages and/or current density on the final density of the resultant ceramics can be attributed to both Joule effect, which increases mass transport by increasing sample temperature, 24,25 and nonthermal effect, which increases mass transport by introducing point defects into the samples. 26 As seen in Table 1, the sample temperatures at flashing stage and steady stage are different. Assuming that for a fixed current density, the sample temperature in the steady stage was a relatively constant.…”

“…As expected, both sample temperature and relative density increase with increasing the initial voltage (samples 1–3) or current density (samples 4–6). The effects of initial voltages and/or current density on the final density of the resultant ceramics can be attributed to both Joule effect, which increases mass transport by increasing sample temperature, 24,25 and nonthermal effect, which increases mass transport by introducing point defects into the samples 26 …”

Flash sintering of high-purity alumina at room temperature

“…Furthermore, the required electric field intensity gradually diminishes with increasing furnace temperature. Yang et al [10] reported that high-purity alumina could be flash sintered at 1300 ℃ by applying an electric field of 5000 Vcm −1 , yielding samples with outstanding mechanical properties. However, it is worth noting that these studies on the FS of high-purity alumina ceramics were conducted under furnace-temperature conditions.…”